1. Field of the Invention
The present invention relates automatic focusing apparatuses and, more particularly, to an automatic focusing apparatus used for a video camera or the like having a zoom mechanism.
2. Description of the Background Art
When a video camera is used for panning photography, because of the change in a distance between the video camera and an object, focusing operation is performed many times, thereby resulting in unclear defocused images with fuzziness. To solve such problem, in Japanese Patent Laying-Open No. 7-77649, for example, a method for detecting an amount of panning by an angular velocity sensor to stop focusing operation has been proposed.
In this case, an output value from the angular velocity sensor is directly used for detecting the amount of panning. If the output from the angular velocity sensor does not return to a zero or reference potential in a prescribed period of time, the angular velocity sensor determines that a camera is in a process of panning and stops the focusing operation or reduces the speed of an AF (Auto Focus) motor.
In an automatic focusing method of the above described conventional example, infrared radiation is emanated so that the focusing operation is performed by the light reflected by an object. There is another example of the automatic focusing method, called a climbing method, in which focusing is performed by a high frequency component of a video signal. In this method, a picture is moved due to panning or lens-shake, so that a spatial frequency and the high frequency component are reduced. As a result, it may erroneously be determined that fuzziness is caused by the change in a distance between a video camera and the object even if there is no change. This is a problem to be solved.
Even with the same amount of panning or shake, if the focal lengths are different, due to the difference in zoom positions, the amounts of movements of the objects on an actual screen appear different in proportion to the focal lengths. Further, when the output from the angular velocity sensor is directly used as in the conventional example, in the case of an angular velocity sensor in which a relatively inexpensive gyro sensor is used, an output value significantly changes with a change in temperature, so that sufficient accuracy is not ensured and a significant cost is required for temperature compensation.
Further, in a more preferred embodiment of the present invention, the lens unit includes a zoom mechanism. A focal length corresponding to a zoom position is detected and the detected focal length is multiplied by the amount of change in angular velocity.
Therefore, a main object of the present invention is to provide an automatic focusing apparatus with enhanced reliability by preventing unstable focusing operation due to camera-shake or panning.
Briefly summarized, the present invention is an automatic focusing apparatus for focusing an image input to an imaging element by controlling a lens unit. In the automatic focusing apparatus, a level of a high frequency component of a video signal obtained from the imaging element is generated by a focus evaluation value generating circuit as a focus evaluation value. Further, a focusing operation is performed such that the focus evaluation value attains a maximum value by changing a relative position of a lens of the lens unit with respect to the imaging element by a focus controlling circuit. When a horizontal rotational movement of the lens unit is detected by a rotational movement detecting circuit and, a value obtained by multiplying the detected amount of change in angular velocity of the lens unit by the focal length exceeds a predetermined threshold value, the focusing operation is stopped by the focus controlling circuit for a prescribed period of time.
Therefore, according to the present invention, when the value obtained by multiplying the detected amount of change in the angular velocity of the lens unit by the focal length exceeds the predetermined threshold value, the focusing operation is stopped for the predetermined period of time, whereby the automatic focusing apparatus with enhanced reliability can be implemented by preventing unstable focusing operation due to camera-shake or panning.
According to a more preferred embodiment of the present invention, a focusing operation is stopped in accordance with a value obtained by multiplying a detected amount of change in angular velocity of a lens unit by a focal length and, the focusing operation is started when a focus evaluation value determined by a focus controlling circuit is kept smaller than a threshold value which is different from the focus evaluation value for a predetermined period of time.